This invention relates generally to a piezoelectric device.
Piezoelectric polymer sensors or piezofilm sensors are increasingly used as piezoelectric sensors. These sensors preferably consist of fluoropolymers, in particular polyvinylidenfluoride (PVDF) and copolymers of PVDF. The piezofilm sensors consist of a piezofilm piece or small plate, on both sides of which is an electrode coating. The electrode coating could be a suitable metal coating, such as silver.
The piezofilm sensor is generally connected to a printed circuit. xe2x80x9cMultilayerxe2x80x9d circuit boards are preferably utilized for this application because of the high circuit density. However, often there is very little space available for the measuring point, or region, out of the film sensor and the board.
The purpose of the present invention is to miniaturize a piezoelectric measuring region. According to the device disclosed herein, a piezofilm sensor is enclosed by two adjacent layers of a multilayer circuit board. The piezofilm sensor is thus completely integrated into the circuit board. Therefore, the space occupied by the piezofilm sensor at the measuring region, is reduced. The result is a significant miniaturization of the measuring region. In addition, the piezofilm sensor is protected against mechanical damage by the circuit board.
In order to obtain a large measuring impulse, the piezofilm sensor fills as large a surface as possible between the adjacent layers of the circuit board. The surface of the piezofilm sensor is thus at least one fifth, preferably at least one third, and most preferably more than half the surface of the circuit board. By configuring the piezofilm sensor as a flexible foil, such a large-surface piezofilm sensor easily withstands deformations caused by a mechanical impulse. Preferably, the circuit board and the piezofilm sensor enclosed therein are essentially rectangular in shape. The thickness of the piezofilm sensor is typically less than the thickness of the adjacent layers of the circuit board. Electric connections are provided at the edge of the piezofilm sensor.
It has been determined that forming recesses in the piezofilm sensor does not noticeably influence the sensitivity of the sensor. Accordingly, the piezofilm sensor of the present invention can include recesses to facilitate through-bonding between the layers of the circuit board without impairing the function of the piezofilm sensor. The recesses can be recesses formed along the edge of the essentially rectangular piezofilm sensor or holes formed within the perimeter thereof. To assure greater sensitivity, the piezofilm sensor is preferably positioned in a recess formed in at least one of the two layers of the circuit board between which it is enclosed.
The piezoelectric device of the present invention could be used in many applications as a measuring device wherein mechanical impulses result in a deformation of the piezofilm sensor. These mechanical impulses could result from any suitable source, such as a hit, vibrations or accelerations. The piezofilm sensor could also be configured as a data-receiving mechanism. Thus, the piezofilm sensor could detect if, when and how an article is subjected to movements, hits or other disturbances.